. Discovery. Science. 238 DISCOVERY sexes, but not directly concerned with the reproduc- tive organs? Here again, recent research has given us a definite answer. The factors necessary for the development of the characters of both sexes are present in all individuals of any of the higher animals, but normallv onlv those which are proper to one sex actually develop. The presence of one or of two X's acts like a switch, which alters the condition in the developing embryo in such a way that in one case only the male characters can reveal themselves, in the other case the female characters. A femal
. Discovery. Science. 238 DISCOVERY sexes, but not directly concerned with the reproduc- tive organs? Here again, recent research has given us a definite answer. The factors necessary for the development of the characters of both sexes are present in all individuals of any of the higher animals, but normallv onlv those which are proper to one sex actually develop. The presence of one or of two X's acts like a switch, which alters the condition in the developing embryo in such a way that in one case only the male characters can reveal themselves, in the other case the female characters. A female animal contains, locked within the chromosomes of her cells, the factors which in other circumstances could com- bine to build a male, the male contains, never expressed in reality, the constitution of a female. That this is so is shown definitely bv the facts observed when two species are crossed. The males of difterent species of pheasants, for instance, differ from each other very much in plumage, while the females are all more or less drab and alike. If :t female Reeves pheasant is crossed with a male of another race, the males among the hybrid offspring show many characters of the male Reeves pheasant,. Fig. i.—.\ feminized M.\LE GUIXE.\ PIG .ACTING .-VS SUCKLING TO A SPECIMEN. By permission of the Editor of the ^'Journal of the Royal Society of ; {After Steiuach.) although these characters must have been transmitted through the chromosomes of the mother, in whom they were invisible. In , the simple presence of one or two X's in the cells of the body is enough to call forth the proper sex-characters; but in higher animals like birds and mammals, there is another link in the chain. This link is furnished bv the reproductive organs. The sex-chromosome machinery acts as a switch which allows either male or female reproductive organs to develop in the embrvo; but as soon as these are developed, they start producing a .secretion or ho
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